Nearly the entire ice sheet covering Greenland—from its thin coastal edges to its two-mile-thick center—experienced some degree of melting for several days in July 2012. According to measurements from three satellites and an analysis by NASA and university scientists, an estimated 97 percent of the top layer of the ice sheet had thawed at some point in mid-July, the largest extent of surface melting observed in three decades of satellite observations.

The data visualization above shows the extent of surface melting in Greenland on July 8 (left) and July 12, 2012 (right). The maps are based on observations from the Special Sensor Microwave Imager/Sounder (SSMI/S) on the U.S. Air Force’s DMSP satellite, from India’s OceanSat-2, and from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua satellites. The satellites measure different physical properties at different scales, and they pass over Greenland at different times. Taken together, they provide a picture of an extreme melt event.

On July 8, satellites showed that about 40 percent of the ice sheet had undergone thawing at or near the surface. By July 12, the extent of melting spread dramatically beyond the norm. In the images above, areas classified as “probable melt” (light pink) correspond to sites where at least one satellite detected surface melting. Areas classified as “melt” (dark pink) correspond to sites where two or three satellites detected melting.

Every summer, a fraction of the surface of the Greenland ice sheet naturally melts. At high elevations, most melt water quickly refreezes in place. Near the coast, some of the melt is retained by the ice sheet and the rest is lost to the ocean.

In mid-July 2012, Son Nghiem of NASA’s Jet Propulsion Laboratory was analyzing radar data from the Indian Space Research Organisation’s Oceansat-2 satellite when he noticed that most of Greenland appeared to have undergone surface melting on July 12. “This was so extraordinary that at first I questioned the result,” said Nghiem. “Was this real or was it due to a data error?”

Nghiem consulted with Dorothy Hall, who studies the surface temperature of Greenland from NASA’s Goddard Space Flight Center. She confirmed that MODIS showed unusually high temperatures over the ice sheet surface and that melt was extensive. Colleagues Thomas Mote of the University of Georgia and Marco Tedesco of the City University of New York also confirmed the melt with passive-microwave data from the DMSP.

The extreme melting coincided with an unusually strong ridge of warm air—a “heat dome”—over Greenland. The ridge was one in a series that dominated Greenland’s weather between May and July 2012.

Even the area around Summit Station in central Greenland, which at two miles above sea level is near the highest point of the ice sheet, showed signs of melting. A National Oceanic and Atmospheric Administration weather station at Summit confirmed that air temperatures hovered above or within a degree of freezing for several hours from July 11 to July 12.

Such pronounced melting at Summit and across the ice sheet has not occurred since 1889, according to ice cores analyzed by Kaitlin Keegan at Dartmouth College. “Ice cores from Summit show that melting events of this type occur about once every 150 years,” said Lora Koenig, a NASA scientist and member of the team analyzing the satellite data. “With the last one happening in 1889, this event is right on time. But if we continue to observe melting events like this in upcoming years, it will be worrisome.”

“The Greenland ice sheet is a vast area with a varied history of change,” said Tom Wagner, NASA’s cryosphere program manager. “This event, combined with other natural but uncommon phenomena such as the large calving event earlier this week on Petermann Glacier, are part of a complex story.”